Radio Shack 26-1061 User manual

Radio /hack
Service m
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MODELIDE
MICROCOMPUTER
Catalog Numbers 26-1061/1062/1063
CUSTOM MANUFACTURED IN U,S,ArBY RADIO SHACkHa DIVISION OF TANDY CORPORATION
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TABLE OF CONTENTS
SECTION PAGE
NUMBER NUMBER
IINTRODUCTION .1
System Overview ...... . 3
II DISASSEMBLY/REASSEMBLY 5
Disassembly •
Reassembly 9
III CPU CIRCUIT BOARD 11
Technical Description 13
Video Sync Circuits ,13
Processor ,.,13
RAM 13
Address Decoding 14
Memory Map 14
10 PORT Description 15
Video 16
Keyboard 17
Cassette Interlace 17
Line Printer and Real Time Clock 18
10 Bus 18
Adjustments and Jumper Positions 18
Model III 10 Bus Detailed Description 19
Model III PORT Bits 21
CPU Module Parts List 25
IV FLOPPY DISK INTERFACE 41
Technical Description 43
Control and Data Buffering 43
Non -Maskable Interrupt Logic 43
Drive Select Latch and Motor On Logic 43
Wait State Generation and WAITIMOUT Logic 44
Clock Generation Logic 44
Disk Bus Selector Logic 44
Read/Write Data Pulse Shaping Logic 44
Disk Bus Output Drives 44
Write Precompensation and Clock Recovery Logic ... 44
Floppy Disk Controller Chip 45
Adjustments and Jumper Options 45
Floppy Disk Interface PC Board Parts List 49
VRS-232C CIRCUIT BOARD 53
RS-232C Technical Description 55
Pin -Out Description 56
Port and Bit Assignments ,57
RS-232C PC Board Parts List .,-62
VI POWER SUPPLY 65
Functional Specifications 67
Troubleshooting 67
Set- Up Procedures 67
Visual Inspection 67
Start-Up 68
No Output 68
Performance Test 69
Power Supply Component Values 70

TABLE OF CONTENTS (Cont'd)
SECTION
NUMBER
VII
VIM
IX
PAGE
NUMBER
VIDEO MONITOR 73
Functional Specifications 75
Service Adjustments 77
ILLUSTRATED PARTS CATALOG 79
Illustrated Parts Catalog Parts List 86
MINI-DISK DRIVE 91
General Description 93
Introduction .,93
Important Notice 93
Physical Description 93
Functional Description 93
Interface Connections ....,.,., 93
Physical Checkout 94
Mounting the Disk Drive 96
Resistor Termination 96
Flat Ribbon Cable Assembly 96
Theory of Operation 97
Introduction -.97
Organization of the Disk Drive :.,,.. 97
Functional Block Diagram Description 97
Index Pulse •.*97
Write Protect 97
Track 00 Switch 97
Spindle Drive 97
Positioner Control ,97
Data Electronics ,93
Data Recording ,.,.,99
Data Reproduction . , 99
Operation 101
Interface Electronics Specifications 101
Input Control Lines 101
SELECT Lines (DS1* -DS4*) 101
Drive Motor Enable (MOTORONl 101
Direction and Step {DIR*} (STEP*) 101
Compensated Write Data (CWD) 101
Write Enable (WG*) 101
Side Select (SDSEL) 103
Output Status 103
Index Pulse (IP*) 103
TRACK 00 (TRK0") 103
Write Protect (WPRT*> .103
Read Data (RD*> 103
Maintenance 105
Physical Description of the PC Boards 105
Circuit Board Test Points 105
Option Select 105
Input Line Terminations 105
Preventive Maintenance 108
Cleaning the Head 108
Alignment and Adjustment 108
Drive Motor Maintenance 108
Carriage Movement Check 108
Head Radial Alignment/CE Alignment 109
Track 00 Alignment 110
Index Sector Adjustment 110
Head Amplitude/Compliance Check 111

TABLE OF CONTENTS (Cont'd}
SECTION PAGE
NUMBER NUMBER
Final Check 111
Disk Controller Board Parts List ,., . ,112
Servo Board Parts List (Non-Linear) 116
Servo Board Parts List (Linear) ,117
Illustrated Parts Catalog Parts List 119
LIST OF ILLUSTRATIONS
FIGURE PAGE
NUMBER TITLE NUMBER
SECTION I-INTRODUCTION
1TRS-80 Model III Block Diagram 4
SECTION III -CPU CIRCUIT BOARD
1CPU Timing Diagram ,,,,13
2Video Timing Diagram ,16
310 Bus Timing Diagram 20
4CPU Printed Circuit Board —Component Side 23
4CPU Printed Circuit Board -Circuit Side 24
5A Model III CPU Schematic Diagram -
Video and Horizontal Sync Circuits 31
5B Model III CPU Schematic Diagram —Processor Section 32
5C Model III CPU Schematic Diagram -RAM Section 33
5D Model III CPU Schematic Diagram —
Address Decoding Section 34
5E Model III CPU Schematic Diagram —Video Section 35
5F Model III CPU Schematic Diagram -Keyboard Section 36
5G Model III CPU Schematic Diagram —
Cassette Interface Section 37
5H Model III CPU Schematic Diagram —
Line Printer and Real Time Clock Section ,38
EJ Model Ml CPU Schematic Diagram -I/O Bus Section 39
SK Model III CPU Schematic Diagram -
RS-232C Interface and FDC Interface Connections 40
SECTION IV -FLOPPY DISK INTERFACE
1FDC Board to CPU Board Signal Description 46
2Floppy Diik Interface PC Board -Component Side 47
2Floppy Disk Interface PC Board -Circuit Side 48
3Model III Floppy Disk Interface Schematic 51

FIGURE
NUMBER
1
2
3
4
LIST OF ILLUSTRATIONS (Cont'd)
PAGE
TtTLENUMBER
SECTION V-RS-232C CIRCUIT BOARD
1RS-232C Board to CPU Board Signal Description 59
2RS-232C PC Board -Component Side ,...60
2RS-232C PC Board -Circuit Side 61
3Model III RS-232C Schematic Diagram 63
SECTION VI -POWER SUPPLY
Test Set-Up •<>7
Q2 Collector Waveform 68
Q2 Base Waveform 68
Power Supply Schematic Diagram 71
SECTION VII -VIDEOMONITOR
1Video Monitor Printed Circuit Board 76
2Deflection Yoke Assembly 77
3Video Monitor Assembly Schematic Diagram 7S
SECTION VII I-ILLUSTRATED PARTS CATALOG
Outer Case Assembly -**1
Case Top Assembly ^2
Base Plate Assembly 83
Metal Chassis Assembly *|4
So
1
2
3
4
5Disk Drive Assembly
SECTION IX -FLOPPY DISK DRIVE
1PC Board Component Locations (Logic Board) 94
1APC Board Components Locations {Servo Boards) 95
2Resistor Termination -^6
3Cable Assembly -Connector Pin Removal Chart ... 96
4
5
6Write Timing Diagram
8Interface Configuration iM
9
10
11 "Cat's Eyes" Pattern :M
12 Radial Adjustment ltJJ
13
14
15
16 Upper Arm and Carriage !
17
13
13
19 Servo Board Schematic 122
20 Mini-Disk Drive Schematic (new board} 123
20 Mini-Disk Drive Schematic (new board) 124
Functional Block Diagram 98
FM Recording S8
Write Timing Diagram 10°
Read Timing Diagram -10°
Interface Configuration 101
Logic Board Test Points and Connector Locations 106
Servo Board Connector Locations 107
Track <WJ Stop Adjustment
Index Sector Timing
Index Adjustment '10
Upper Arm and Carriage '
1'
Illustrated Parts Catalog -Exploded View 118
Mini-Disk Drive Schematic (Sheet 1) •120
Mini-Disk Drive Schematic (Sheet 2) 121

LIST OF TABLES
TABLE PAGE
NUMBER TITLE NUMBER
SECTION V-RS-232C CIRCUIT BOARD
BRG Programming Table ,,..,... 65
SECTION VI -POWER SUPPLY
1Load Board Values gg
2Voltage and Ripple Specifications ,,.,69
SECTION IX -MINI-DISK DRIVE
1Interface Connector Pin Assignments 102
2Power Connector Pin Assignments 102

SECTION I
INTRODUCTION

SYSTEM OVERVIEW
The Radio Shack TRS-80 Model III Microcomputer is an enhanced version of Radio Shack's popular TRS-80 Microcom-
puter (Model I). The TRS-80 Model III is software compatible with the Model Iso that owners of either system can take
advantage of the large number of programs available.
Features of the TRS-80 Model III which are common to the TRS-80 Model Iinclude:
•Availability of Level Ior Level II BASIC in ROM
•Full size typewriter style keyboard
•A12-inch video display
•Built-in cassette interface
•Character display of 16 lines of 64 characters
•Graphics under control of BASIC (128 Hx48 V)
•ULrecognized construction
In addition, the Model III has the following standard features:
•12-key numeric keypad for rapid entry of numbers
•Rugged cabinet housing keyboard, electronics, video display, and power supply
•Direct drive video monitor for Improved resolution
•Internal power supply
•Parallel printer port for use with Radio Shack printers
Several other features are available when Level II BASIC is used; features such as: real time clock, upper and lower case
characters, RAM internally expandable to 4SK bytes, I/O port for peripheral expansion, and cassette interface available
with 500 and 1500 baud rates.
Optional peripherals for the TRS-80 Model III include disk drives (two built-in, two external) with double density for
increased storage capacity, and abmtr-in RS-232 serial interface for communications and peripheral interface,
The Block Diagram shows the various internal components and connections of the Model III Microcomputer.

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SECTION II
DISASSEMBLY/REASSEMBLY
5

DISASSEMBLY
CASE: RS-232 BOARD (optional):
1.Remove all cables from the bottom and rear of the Com-
puter. Position the Computer on its rear panel to provide
easy access to the case bottom. Remove the ten screws
from the case bottom. Notice the different types and
lengths of screws and note their positions. Set them
aside in groups.
2. Position the Computer upright and remove the #6 screw
and washer from the top of the back panel of the case.
3. Very carefully remove case top, lifting striaght up and
setting it aside to the left (if facing video screen). Be
careful not to exceed the length of the video cable,
4. Remove screws from the chassis shield and The ground
connectors and remove the shield. **
CPU BOARD:
1. Remove all cables connecting the CPU Board (power
supply cable, video, keyboard, and cassette cables, and
if applicable, the RS-232 and FDC inter-connect cables).
2. Remove the five screws fastening the PC Board (two in
upper right corner, three on bottom],
3. Remove the small PCB Mount Bracket and its two
screws from atop the metal chassis bracket. **
4. Make sure all cables to the CPU Board have been discon-
nected then remove the Board. (If your unit uses plastic
spacer mounts to hold the Board, press the small tabs on
the mounts through the mounting holes irr the PCB and
gently pull the CPU Board off.)
FDC BOARD (optional):
NOTE: The CPU Board must be removed before removing
the RS-232 Board.
1. Be sure to remove all cables connecting the RS-232
Board.
2. Remove the screws connecting the PC Board to the
chassis and remove the Board
.
MAIN POWER SUPPLY:
1.Remove all interconnect cables to the Boards fastened
on the metal chassis bracket (power supply cable,
video, keyboard, and cassette cables, and if applicable,
RS-232 and FDC power connectors and the Disk ribbon
cable).
2. Remove the four screws fastening the metal chassis
bracket to the case bottom.
3. Make sure all inter-connecting cables are removed, then
carefully lift out chassis bracket and attached Boards.
4. Remove the four screws fastening the Power Supply to
the Bracket (notice where the ground tab is fastened),
then carefully remove the power supply.
DISK DRIVE POWER SUPPLY (optional):
1
.
Disconnect all cables and wires connecting to the Power
Supply.
2. Remove the four screws fastening the Board to the Disk
Mounting Bracket (notice where the ground tab is fas-
tened), then carefully remove the Power Supply.
NOTE: The CPU Board must be remove before removing the
the FDC Board.
1. Be sure to disconnect all cables to the FDC Board,
2. Remove the screws holding the FDC Board to the metal
chassis and remove the Board,
DISK DRIVES (optional):
1- To remove the Disk Drive in the top position, carefully
remove the FDC inter-connect cable connected to the
rear of the Drive.
2. Remove the four screws and washers (two on each side)
which connect the Drive to the Disk Mounting Bracket.
3. Disconnect the power supply connector from the bot-
tom of the top board in the Disk Drive and also remove
the ground wire from the rear of the Drive.
"Not applicable to all units.

4. To remove the Disk Drive In the bottom position, you
must first remove the Disk Drive Power Supply.
5. After removing the Power Supply, remove the FDC
inter connect cable from the rear of the Drive.
6. Remove the four screws and washers (two on each side)
which connect the Drive to the Disk Mounting Bracket.
7. Disconnect the power supply connector from the bot-
tom of the top board in the Disk Drive and also remove
the ground wire from the rear of the Drive.
VIDEO MONITOR (CRT) AND VIDEO BOARD
1
.
Disconnect the four color coded wires with spade lugs
from the CRT yoke. (Be sure to note their positions.)
2. Disconnect the connector on the rear of the CRT neck.
WARNING
There may be ahigh voltage charge on the high voltage
anode. To discharge, connect one end of awire to a
known good ground and connect the other end of the
wire to the blade of acommon screwdriver. Insert the
screwdriver blade under the suction cup and touch it
to the clip holding the wire to the CRT.
3, Insert a common screwdriver under the suction cup on
the high-voltage anode wire on the side of the CRT. Use
the screwdriver to compress the clip holding the wire to
the tube and pull the wire free.
4, Remove the ground wire fastened directly to the Video
Board.
5, Remove the upper right and tower left nuts and washers
which hold the CRT in place.
CAUTION
If dropped, the CRT may implode. To avoid this kind of
accident, support the CRT while performing the next
step.
$. Remove the remaining lower right and upper left nuts,
and washers and carefully remove the CRT,
7. Disconnect the CPU cable connector from the Video
Board
.
8. Remove the two screws fastening the Video Board to the
Case Top and carefully lift out Board.
8

REASSEMBLY
RS-232 BOARD (optional):
1. Install the PC Board using 4j& x3/8" screws, If appli-
cable, press the PC Board onto the plastic spacer mounts
then fasten with the screws.
2. Reconnect all cables to the RS-232 Board.
FDC BOARD (optional):
1. Install the PC Board using #6 x3/8" screws and the
plastic spacer mounts, if used.
2. Reconnect all cables to the FDC Board.
CPU BOARD:
1. Make sure good insulating washers are attached to the
CPU Board (Rev. Fboards only) then fasten the Board
jsing #6 x1/4" screws.
2. Reconnect all cables to the CPU Board (power supply
cable, video, keyboard, and cassette cables, and if appli-
cable, RS-232 and FDC inter-connect cables).
3. Attach the small PCB Mount Bracket (if used) to the
metal chassis bracket with two #6 x1/4" screws.
6. Reconnect the FDC interconnect cable to the rear of
the Drive,
DISK DRIVE POWER SUPPLY (optional]
:
1. Before installing the Power Supply, be sure that the
bottom Disk Drive is mounted in place and the Disk
Shield is in positon on the Disk Mounting Bracket.
2. Reconnect all cables and wires to the Power Supply.
3. Fasten the Power Supply with four #6 x3/8" screws.
Be sure the ground tab is fastened back in place.
VIDEO MONITOR fCRT) AND VIDEO BOARD:
1
.
Position the CRT in the Case Top and install the upper
left and lower right #10 washers and nuts.
2. Install the upper right and lower left #10 washers and
nuts. Be sure to reconnect the ground wire from the
CPU cable. It will require two nuts to fasten it.
3. Install the Video Board into the Case and fasten with
two #6 x1/4" screws.
4. Connect the ground wire with solder lug back to the
Video Board,
MAIN POWER SUPPLY:
1. Fasten the Power Supply to the metal chassis bracket
using four #6 x1/4" screws. Be sure the ground tab is
fastened back in place.
2. Install the chassis bracket (with all Boards in place) in
the case bottom using four #6 x1/4" screws.
3. Reconnect all cables (power supply, video, keyboard,
and cassette cables, and RS-232 and FDC power cables
and Disk ribbon cable if necessary),
DISK DRIVE (optional):
1. Place the Disk Drive in the bottom position and recon-
nect the ground wire and power supply connector,
2. Fasten the Drive with four #6 x1/2" screws and four
flat washers (two on eacrt side).
3. Reconnect the FDC inter-connect cable to the rear of
the Drive.
4. Position the second Disk Drive in the top position and
reconnect the ground wire and power supply connector.
5. Fasten with four screws and washers (two on each side).
5. Install the plug on the rear of the CRT neck.
6. Install the four color coded wires with spade lugs to
their associated terminals (as determined by acolored
dot on the yoke near each terminal).
7. Install the high-voltage anode wire on the side of the
CRT. Use ascrewdriver to Compress the clip and insert
it into the CRT. Press down on the suction cup to
secure.
CASE:
1. Double-check to be sure all wires are connected cor-
rectly and all Boards are properly fastened.
2. Attach the chassis shield (if used) with #6 x1/4"
screws and reconnect the ground connectors.
3. Carefully place the Case Top over the Case Bottom.
Do not hit the CRT neck. It could implode or break
off.
4. Install the #6 x3/8" sheet metal screw and flat washer
in the top rear panel of the Case.
5. Carefully rest the Computer on its rear panel and replace
the ten #8 screws; five 1" sheet metal toward rear, three
7/8" machine head along front, and two 1" machine
head in remaining positions.
9

SECTION III
CPU CIRCUIT BOARD
11

TECHNICAL DESCRIPTION
The technical description of the Model III Computer CPU
circuit board will be broken down into nine sections. These
are;
1. Processor
2. RAM
3. Adress Decoding
4. Video
5. Video Sync Circuits
6. Keyboard
7. Cassette Interfaces
8. Line Printer and Real Time Clock
9. I/O Bus
This breakdown, which follows the partitioning of the sche-
matic diagrams, will allow easy explanation and referencing.
VIDEO SYNC CIRCUITS
The video sync circuits are required to meet the pulse width
and polarity requirements of the video monitor. It also allows
adjustment in both the horizontal and vertical planes in dis-
crete increments. The vertical sync pulse VSYNC is active
low and approximately 693/«ec in duration. The frame rate
is either 50 or 60 Hertz, The horizontal rate is 15,840 Hz and
the signal HSYNC is active high with aduration of approxi-
mately 8,usec. Tne vertical plane adjustment covers a total
of eight rows and is adjustable in increments of one row
(R1, R2, R4). The horizontal plane adjustment increments
by two characters for a total coverage of 16 character posi-
tions. Both adjustments are accomplished with the use of
wired "AND" gates U22 and U38 {LS266J.
PROCESSOR
The CPU chip is aZ80 that runs at aclock speed of 2.02752
MHz:. The CPU clock is derived by dividing the basic video
clock (10.1376 MHz) by five. U62 performs this function
and the clock is non-symmetrical with a40 percent high duty
cycle. The clock signal PQC is run through an active pull-up
and becomes PCLOCK which has afull B-volt swing with fast
rise and fall times. The reset switch, which is located on the
keyboard, is ORed with power on reset (R7, C54, U15) to
provide aSystem RESET* signal. The Reset pin on the Z-80
is driven by RESET*. The address lines are buffered by
LS244*s {U91, U92), the data lines by an LS245 (U90),and
control lines buffered by an LS367 (U76), The buffered
control lines are combined in U89 and U86to form memory
and IO port controls (RD*, WR*, IN*, OUT*).
See the CPU Timing Diagram for exact time relationships.
U75 is used to switch the data bus buffer during (NTAK*
cycle or any read operation (IO or Memory), Whenever one
of the ROM's is being accessed, the data bus buffer is dis-
abled by U108 since the ROM's data lines are located on the
CPU data bus. BUSRQ*, HALT*, BUSAK*, and RFSH*are
not used. The Model III has three sockets for the ROMs
U104, U105, U106). They are 64K bit,32K bit and 16Kbit
ROMs, respectively.
RAM
The RAM consists of an array of up to twenty-Four 16K
dynamic memory chips with damping resistors located on all
address and control lines (RP1 ,RP2, RP3), Chips U7 to U14
are addressed at 4000 to 7FFF, U25 to U32 are addressed at
8000 to BFFF and U43 to U50 are addressed at C00O to
FFFF. The memory data is interfaced to the data bus by
two 8T26; (U63, U64), Normally the data is driven into the
memory array. However, on aread cycle U19 switches the
direction of the transceivers and drives memory data onto
the data bus. The 5volts required by the RAMs is supplied
by a3-terminal regulator (MC7905C) which takes -12 volts
as its input. See the CPU Timing Diagram for the relationship
between RAS*, MUX, CAS*.
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FIGURE!. CPU TIMING DIAGRAM
13